1. Field of the Invention
The present invention relates to a printed circuit board having electronic parts mounted at a high density and a method of manufacturing the same. More particularly, the present invention relates to a structure of a printed circuit board and a method of mounting integrated circuit (IC) packages, a tape automated bonding (TAB) type of tape carrier package (TCP) and the like on the printing circuit board in a surface mounting manner.
2. Description of Related Art
Conventionally, soldering has widely been used to mount electronic parts on printed circuit boards. In the field of mounting by soldering, with the advancement of miniaturization and combination of parts based on the development of large-scale semiconductor IC integration techniques, the method of mounting package type and bare chip type IC parts in a surface mounting manner has come into major use replacing the early-developed method of mounting parts with leads such as single-in-line packages (SIP) and dual-in-line packages (DIP) by inserting the leads in a circuit board. Such surface mounting is advantageous in terms of assembly automatization and electronic characteristics. In particular, TCPs have a large chip area and are suitable for a multi-pin arrangement. TCPs also have the advantage of being mountable at a high density because the lead pitch of the TCPs can be reduced.
Surface mount devices (SMD) are usually connected to circuit boards by reflow soldering. Reflow soldering is performed in such a manner that a SMD is set on solder layers previously formed on electrode pads (lands) on a circuit board, and the solder layers are melted by being heated in a reflow furnace to fix the SMD to the printed circuit board. On the other hand, TCPs are ordinarily connected in such a manner that outer leads of a TCP are pressed with a bonding tool against solder layers previously formed on lands on a circuit board, and the solder layer and/or outer leads are heated by a local heating device while being pressed against each other. This is because the outer leads of TCPs are very soft and deformable. Such solder layers are formed, for example, by printing cream solder on the circuit board where no parts are mounted.
To improve the wettability of solder, flux is ordinarily used. Recently, from the viewpoint of earth environmental protection, use of water-soluble or cleaning-free type flux for soldering has been increased. Alternatively, alloying by thermocompression bonding, a method of using an anisotropic conductive sheet or a photo-curable insulating resin and other methods are used instead of soldering to mount the electronic parts.
According to the above-described conventional mounting methods, if one or more SMDs and TCPs are mounted on one circuit board, a process step for connecting the TCP is ordinarily performed after a reflow step for connecting the SMD. In such a case, as shown in FIG. 12, solder layers 31 located on lands 35 to which a TCP are to be connected are melted by the preceding reflow step so as to round into a semispherical shape by the effect of surface tension. The outer leads 32 of the TCP placed on the solder layers 31 may slip down leftward or rightward as shown in FIG. 12 when pressed by a bonding tool 33, so that the outer leads 32 are not accurately bonded to the corresponding lands 35, resulting in occurrence of connection failure, i.e., short-circuit or the like, even if the pressing direction is perpendicular to circuit board 34. Therefore, there is a problem of a reduction in yield and, hence, an increase in manufacturing cost. Positioning of the outer leads of a TCP on a circuit board becomes more difficult and a misalignment of the leads can occur more easily if the pitch between the outer leads becomes smaller with the increase in the integration density or the density of ICs. If a special positioning means or fixing means is used to cope with this problem, then a troublesome operation is required to use such means; the amount of labor and time taken for positioning is increased and the manufacturing cost is also increased.
The development of SMDs having a relatively large number of pins and a relatively small pin pitch have also been promoted. In the case of simultaneously mounting a multiplicity of SMDs on a circuit board in a surface mounting manner, it is difficult to accurately position the leads of the devices, and there is a substantially high probability of occurrence of solder bridging and misalignment. Further, if SMDs and TCPs are both mounted on a circuit board, the total number of mounting steps is increased and the time required for mounting is relatively long, because the SMDs and TCPs are mounted by separate connecting steps.
Leads of electronic parts may be soldered to lands on a printed circuit board, for example, in a manner described below. First, as shown in FIG. 13A, a dispenser 36 is used to supply flux 37 to a region including a land 35 on the printed circuit board. If the flux is a water-soluble or cleaning-free type flux as mentioned above, it may spread only to such an extent as to partially cover the surface of a resist layer surrounding the land 35, since the viscosity of the flux is low. Therefore, there is a possibility of failure to sufficiently immerse a connected end portion of a lead 32 placed on a solder layer 31. If the connected end portion of lead 32 is pressed and heated by a bonding tool 39, as shown in FIG. 13B when not sufficiently immersed in the flux 37, the connected end portion of lead 32 will not be sufficiently wetted with the solder. In such a situation, the probability of occurrence of connection failure is high and the yield is reduced. In particular, in the case of a part such as a TCP having a multiplicity of leads arranged with a very small pitch, a troublesome correction operation and a long time are required to correct even a connection failure of only one lead.
When leads and lands are pressed against each other with a bonding tool as described above, it is important to maintain a certain parallelism between the bottom surface of the bonding tool and the surface of the printing circuit board. As shown in FIG. 14, if a bottom surface 40 of a bonding tool 39 presses leads 32 of electronic parts in a state of being inclined with respect to a circuit board 34, there is a possibility of the leads slipping down solder layers 31 of corresponding lands 35 causing connection failures or short-circuits in the wiring.
If the lead pitch is very small, as in the case of a TCP, a misalignment can occur easily between the leads and lands even if the inclination of the bonding tool is small. For example, in a case where a through hole is formed in the vicinity of a land or a material is attached to an applied layer of a resist 38 on a wiring pattern 41 by silk screen printing or the like so as to form a character 42 or the like, the bottom surface 40 of the bonding tool may abut one-sidedly against the protrusion of the character 42 or the like so that the desired parallelism is lost if the height of the character or the like is larger than that of the lands by an amount such as 20 mm. If a multiplicity of leads 32 and corresponding lands 35 are arranged with a small pitch, the bonding tool may be replaced by a different bonding tool having a bottom surface size and a bottom surface configuration conforming to the lead arrangement to avoid the above-described problem of parallelism. In such a case, however, a universality of the bonding tool is lost; a need for changing and positioning a bonding tool for each process step arises as well as a need for preparing various types of bonding tools. A large amount of labor and time are thereby required, resulting in a considerable increase in manufacturing costs.
On a printed circuit board, a plurality of marks for accurately positioning electronic parts which are to be connected to the printed circuit board may be provided. Such positioning marks are sometimes formed by portions of wiring patterns, but are ordinarily provided on a circuit board as marks independent of wiring patterns. Japanese Patent Laid-Open Publication No. 60-161693 discloses a printed circuit board arranged with such recognition marks provided at arbitrary positions on the board, particularly in the vicinity of mount portions of electronic parts so that the electronic parts can be accurately positioned. However, if such positioning marks are provided only at arbitrary positions, a need for increasing the area of the printed circuit board arises and it is difficult to increase the density of mounted parts and the density of wiring. Moreover, the circuit wiring becomes complicated and the freedom of circuit design is reduced, so that the circuit design cannot be sufficiently adapted for miniaturization of the circuit board and semiconductor devices. It is also necessary for the positioning marks to be formed so as to be clearly recognizable, because the positioning marks are optically detected by a sensor.